Licensed Assisted Access (LAA) facilitates Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) equipment to operate in the unlicensed 5 gigahertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Devices can connect in the licensed spectrum (using a Primary Cell (PCell)) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum (using a Secondary Cell (SCell)). To reduce the changes involved for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously used in the SCell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) procedure needs to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.” In many regions there is also a constraint on the maximum duration of a single transmission burst in the unlicensed spectrum, such as 4 milliseconds (ms) or 10 ms.
1. LTE
FIG. 1A illustrates a basic LTE downlink physical resource grid. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM (DFT-spread OFDM), which is also referred to as single-carrier Frequency Division Multiple Access (FDMA), in the Uplink (UL). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1A, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The duration of each symbol is approximately 71.4 microseconds (μs). The UL subframe has the same subcarrier spacing as the Downlink (DL) and the same number of Single Carrier FDMA (SC-FDMA) symbols in the time domain as OFDM symbols in the DL.
FIG. 1B illustrates an LTE radio frame. In the time domain, LTE DL transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length TSUBFRAME=1 ms as shown in FIG. 1B. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. A subframe is divided into two 0.5 ms slots. For normal cyclic prefix, each slot consists of 7 OFDM symbols. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one 0.5 ms slot in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
FIG. 1C illustrates an example LTE 1.0 ms subframe (with 14 OFDM symbols) showing the locations of control signals and reference signals. DL transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current DL subframe. This control signalling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The DL subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A DL system with CFI=3 OFDM symbols as control is illustrated in FIG. 1C.
From LTE Release 11 (Rel-11) onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For LTE Rel-8 to Rel-10, only the Physical Downlink Control Channel (PDCCH) is available. The reference symbols shown in FIG. 1C are the Cell specific Reference Symbols (CRSs) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
1.1 PDCCH and EPDCCH
The PDCCH/EPDCCH is used to carry Downlink Control Information (DCI) such as scheduling decisions and power control commands. More specifically, the DCI includes:                DL scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, Hybrid Automatic Repeat Request (HARQ) information, and control information related to spatial multiplexing (if applicable). A DL scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of HARQ acknowledgements in response to DL scheduling assignments.        UL scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and HARQ-related information. A UL scheduling grant also includes a command for power control of the PUSCH.        Power control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both DL and UL simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio channel conditions, link adaptation can be used, where the code rate of the (E)PDCCH is selected by adapting the resource usage for the (E)PDCCH, to match the radio channel conditions.
1.2 CA
FIG. 2 illustrates an example of CA. The LTE Rel-10 standard supports bandwidths larger than 20 megahertz (MHz). One important aspect of LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments, it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to ensure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of CA. CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least have the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 2. A CA-capable User Equipment device (UE) is assigned a PCell which is always activated, and one or more SCells which may be activated or deactivated dynamically.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more DL CCs than UL CCs, even though the cell is configured with the same number of UL and DL CCs.
In addition, a key feature of CA is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH is also configured semi-statically. Note that cross-subframe cross-carrier scheduling of PDSCH is not supported in Rel-11 CA, i.e., the (E)PDCCH grant in a particular subframe applies to a PDSCH allocation in that same Transmit Time Interval (TTI).
2. WLAN
FIG. 3 is a general illustration of an LBT mechanism. In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for medium access. The channel is sensed to perform a Clear Channel Assessment (CCA), and a transmission is initiated only if the channel is declared idle. If the channel is declared busy, the transmission is essentially deferred until the channel is deemed to be idle. When the range of several Access Points (APs) using the same frequency overlap, transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. If several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
3. LAA to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE (i.e., licensed spectrum). This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and, as such, cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi, as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. As used herein, an SCell in unlicensed spectrum is denoted as an LAA SCell. In the case of cross-carrier scheduling, PDSCH and PUSCH grants for the LAA SCell are transmitted on the PCell.
Another way to utilize the unlicensed spectrum is to utilize standalone LAA cells.